Many philosophers claim that it cannot be the case that a person ought to perform an action if this person cannot perform this action. However, most of these philosophers do not give arguments for the truth of this claim. In this paper, I argue that it is plausible to interpret this claim in such a way that it is entailed by the claim that there cannot be a reason for a person to perform an action if it is impossible that (...) this person will perform this action. I then give three arguments for the truth of the latter claim, which are also arguments for the truth of the former claim as I interpret it. (shrink)